(1). Field of the Invention
The present invention relates to a process for preparing a nitride film present in various semiconductors, and more specifically, it relates to a process for preparing a nitride film by a chemical vapor deposition method or the like. The chemical vapor deposition method (the CVD method) is a technique which comprises feeding a raw material in a gaseous state, and then carrying out a chemical reaction in a vapor phase or on the surface of a substrate to deposit a thin film thereon. According to this method, the formation of various thin films is possible in a range wider than other deposition methods such as a vacuum vapor deposition method and a sputtering method, and for this reason, the chemical vapor deposition method is used to manufacture semiconductor films, insulating films, metallic films and the like which are necessary for the manufacture of semiconductor devices.
(2). Description of the Prior Art
In a chemical vapor deposition method, a material gas is selected in accordance with a kind of thin film to be deposited, but in general, there can be used a material which is gaseous at ordinary temperature as well as a liquid and a solid material having a sufficiently high vapor pressure. In practice, a material which is easily available in a high-purity state and free from toxicity and the danger of explosion is selected, and for the purposes of improving the quality of the thin film, lowering a deposition temperature and accelerating a deposition speed, novel material gases have been developed.
Heretofore, as the material gases which are nitrogen sources for use in the manufacture of the nitride film by the chemical vapor deposition method, there can usually be used ammonia, hydrazine (NH.sub.2 NH.sub.2), monomethylhydrazine (CH.sub.3 NHNH.sub.2) and dimethylhydrazines [(CH.sub.3).sub.2 NNH.sub.2 ] and (CH.sub.3)NHNH(CH.sub.3)]. In addition, as another material gas which can react with the material gas as the nitrogen source to form the nitride film, there can be used compounds of silicon, titanium, aluminum, tantalum, tungsten and the like. Typical examples of a technique for manufacturing the nitride film include a high-temperature thermal CVD method, a plasma CVD method and a photo assisted CVD method, and a suitable manufacturing method is selected from them in consideration of a use purpose of the nitride film.
As an example in which ammonia is used as the material gas of the nitrogen source, a method is well known which comprises reacting the ammonia gas with a gas of silicon hydride or a gas of silicon halide to from a silicon nitride film. According to this manufacturing method, an Si.sub.3 N.sub.4 film having an excellent crystallinity can be formed. However, since ammonia has a high decomposition temperature, the deposition temperature of the nitride film is as high as about 1000.degree. C., so that the escape of nitrogen from the deposited nitride film or a lamination defect in the nitride film tends to occur. In addition, it is impossible to do wiring on a substrate by the use of a metal having a low melting point such as aluminum, and it is also impossible to deposit the nitride film on the substrate which cannot withstand a high temperature.
In the plasma CVD method, since the reaction takes place in a plasma state having a high energy, the deposition temperature of the nitride film can be set to about 400.degree. C., which permits the remarkable drop of the deposition temperature. In this case, however, the composition of the nitride film is such as to be represented by Si.sub.x N.sub.y, so that the denseness of the nitride film is lower than that of the nitride film manufactured by the high-temperature thermal CVD method. Moreover, also in the case of the photo assisted CVD method, radicals which contribute to the reaction are produced by light irradiation, and therefore the drop of the deposition temperature is possible, but similarly to the plasma CVD method, the deterioration of the denseness of the nitride film cannot be avoided.
Furthermore, as an example using the chemical vapor deposition method, there is a method in which a GaN-containing compound semiconductor is manufactured by a reaction of ammonia with an alkyl compound of gallium or the like. However, since the decomposability of ammonia is low, a high V/III ratio of 1000 to 20000 (a ratio of mols of nitrogen in the group V of the periodic table/mols of a metal in the group III thereof) and a high nitride film deposition temperature of about 1000.degree. C. are required. In consequence, the escape of nitrogen is liable to occur, or there is a problem that, for example, an element In is scarcely introduced into the deposited layer during the deposition of an InGaN film.
In recent years, it has been investigated to use hydrazine, monomethylhydrazine and dimethylhydrazines as the nitrogen source gases for the manufacture of the nitride film. These compounds are liquid at ordinary temperature, but they have a sufficiently high vapor pressure and hold an N--N bond in the molecule which is decomposable at a lower energy than an N--H bond. Therefore, they have characteristics that the N--N bond is cleaved at a temperature lower than the decomposition temperature of ammonia and hence the decomposition occurs. Accordingly, in the case that any of these compounds is used as the material gas of the nitrogen source, the nitride film can be deposited even at a nitride film deposition temperature of about 500.degree. C., and therefore the manufacture of the nitride film is possible even at a temperature of about 500.degree. C. In addition, the nitride film having a satisfactory quality can be obtained.
However, hydrazine and monomethylhydrazine are recognized to have a variant which induces a mutation of a gene, and dimethylhydrazines also have characteristics that a TLV (an allowable concentration) regulated by ACGIH is very low. Thus, with regard to these compounds, much attention has been paid to their extremely high toxicity.
On the other hand, in addition to the above-mentioned chemical vapor deposition method, another technique for preparing the nitride film is known and utilized. For example, a silicon substrate can be held at a high temperature in the material gas atmosphere of a nitrogen source such as ammonia or hydrazine to directly nitride Si, thereby forming an SiN layer on the surface of the silicon substrate. Similarly, it is also possible to form an SiON layer on the surface of an SiO.sub.2 substrate by the use of ammonia, hydrazine or the like. In any of the above-mentioned methods, however, the problems of the toxicity, the vapor pressure and the decomposition properties regarding the material gas of the nitrogen source cannot be avoided.